Î³-Fe2O3 Charge Transfer Interface toward ... - ACS Publications

Subscriber access provided by READING UNIV

Article

ZnO/#-Fe2O3 charge transfer interface towards highly selective H2S sensing at a low operating temperature of 30#C Sugato Ghosh, Deepanjana Adak, Raghunath Bhattacharyya, and Nillohit Mukherjee ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00636 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sensors is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

ZnO/γ-Fe2O3 charge transfer interface towards highly selective H2S sensing at a low operating temperature of 30⁰C

Sugato Ghosh, Deepanjana Adak, Raghunath Bhattacharyya and Nillohit Mukherjee* Centre of Excellence for Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India *Corresponding author email: [email protected] Fax: +91-33-2668-2916

Abstract: ZnO/γ-Fe2O3 heterostructure has been deposited in the form of thin films using a single step facile electrochemical technique. Considering the unique properties of both ZnO and γ-Fe2O3 towards the sensing of reducing gases, the concept of forming a heterostructure between them has been conceived. The structural characterization of the deposited material have been performed using x-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy; which revealed a flower-like morphology with the co-existence of both ZnO and γ-Fe2O3 leading to the formation of a heterostructure. The material showed excellent sensing properties towards the selective detection of H2S at room temperature (30ºC) among the three test gases, viz. CH4, H2S and CO. The effect of relative humidity was also studied to have an idea about the performance of the device under real situation. The results are promising and better than many commercially available sensors. The room temperature selective detection will help in facile fabrication of portable gadgets.

Keywords: ZnO/γ-Fe2O3 heterostructure; gas sensor; hydrogen sulfide; selective detection; room temperature; humidity dependence

1 ACS Paragon Plus Environment

ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Recently, there has been a huge environmental concern over the numerous toxic gases found in the confined places such as manholes, coal mines, oil and natural gas industries, which are causing threat to the persons working there. Thus, a great deal of research has been focused on developing highly sensitive and selective gas sensors that can detect hazardous gases like methane, carbon monoxide, hydrogen sulfide, ammonia, etc., with short response and recovery time and most importantly, work in low operating temperature. High operating temperature of the device will require complex sensing platform and may lead to explosion in the case of explosive gases present the confined area. On the other hand, low operating temperature will help in facile fabrication of portable gadgets with minimum probability of explosion. Hydrogen sulfide (H2S) is one of the most abundant toxic gases [1] found in the manholes, coal mines and natural gas industries which is generated due to the prokaryotic breakdown of organic matter in the absence of oxygen. The presence of H2S in low concentration (10-50 ppm) can be noticed by its pungent smell, however, at high concentration (>1000 ppm) it blocks our olfactory lobes and it becomes difficult to detect its presence by smelling. At high concentration, the gas is extremely toxic which affects eye, respiratory and central nervous system and can cause pulmonary edema leading to death [2]. On the other hand, the auto-ignition temperature of H2S is 232ºC and the gas has the lower explosion limit (LEL) of 4.3%. In most of the cases, H2S coexists with other two reducing gases like methane (CH4) and carbon monoxide (CO), the former is explosive and the later is toxic. As all the three gases are reducing in nature, it has become essential to develop a material which can detect H2S selectively among the three at a very low operating temperature, preferably at room temperature (30ºC). During past couple of decades, researches haves already been carried out in developing H2S sensors based on metal oxide semiconductors (MOS) like SnO2, ZnO, TiO2, WO3, Fe2O3 and In2O3 [3]. Some functional


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

materials, like graphene [4] and carbon nanotube [5] have also been exploited as gas sensors due to their unique electronic properties. The MOS based gas sensors have received special attention because of their ease of fabrication, low manufacturing cost, low operating power, stability and ease of incorporation with microelectronic devices with possible high device integration density. On the other hand, the metal oxide semiconductors can be prepared with unique structures like one dimensional nanowire, nanorod, nanobelt and two dimensional structures like nanoplates, sheets and flakes [6]. These structures have high surface-area-to-volume ratio, resulting in enhanced surface reactivity and increasing gas accessibility, thereby making them suitable candidate for detection of gases [7]. Among various MOS, ZnO and Fe2O3 are most attractive gas sensing materials due to their high response, ease of fabrication with tunable morphologies leading to high surface-area-to-volume ratio, chemical stability and low response and recovery time [8]. Wang et al. have reported ZnO nanorod based H2S gas sensor at room temperature [9], however, the response time of their sensor is huge (20 minutes) and the response curve never recovered to the original baseline after degassing, which indicates lack of repeatability. Bodade et al. have demonstrated ZnO-TiO2 based H2S gas sensor with different mol% of CdO incorporation [10]. In their case, the device was functional only at an operating temperature as high as 250°C. Zhu et al. [11] has demonstrated porous ZnO-nanosheet built network based H2S sensor, though significant response was observed only at higher operating temperature of 200°C and no data on selectivity has been reported. Most of these materials suffer from limitations in terms of achieving response and selectivity while operating at room temperature. Very often these metal oxide semiconductors are used with noble metals like Au, Pt and Pd to achieve enhanced response [12,13] by surface fuctionalization, however, in most of the cases, they suffers in lack of selectivity. The use of metal alloy like Pd-Ag as an alternative of a single metal


ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

to enhance the selectivity as well as sensor response at a reasonably low operating temperature has been reported by us elsewhere [12]. There are also several reports on gas sensing by composite materials like ZnO-CuO, ZnO-In2O3, SnO2-CuO and ZnO-ZnO2, to name a few [14,15]. Wang et al. [16] have reported CuO nanoparticle decorated porous ZnO based H2S sensor, but the major drawback was long sensing and recovery time. Park et al. [17] have fabricated H2S sensors based on In2O3-core/ZnO-shell nanorods with an operating temperature of 300°C and the device was tested at a very low concentration of 10–100 ppm. The enhanced gas sensing property of these composites can be attributed to various electronic, chemical and structural factors like favorable band bending due to Fermi level equilibration, charge carrier separation, decrease in activation energy, enhanced catalytic activity, surface area enhancement and increased gas accessibility [18]. In this work, for the first time we are reporting ZnO/γ-Fe2O3 heterostructure based highly sensitive H2S sensor that can selectively detect H2S in presence of other reducing gases like CO and CH4 at room temperature (30°C). Almost in all cases, the materials were found to show either n-type response (resistance decrease in presence of target gas) or p-type (resistance increase in presence of target gas) response for all its target gases, but it is rare to find out a single material behaving as n-type in presence of a particular gas, and p-type in presence of other gases at room temperature. Interestingly, the prototype reported here, have shown n-type response in presence of H2S gas and p-type response in presence of CO and CH4. The probable sensing mechanism with reactions involved has also been proposed for the selective detection of H2S. For real life application of the sensor prototype, detailed study on the influence of relative humidity on the sensing performance has also been carried out.


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

Experimental Chemicals Zinc sulphate heptahydrate (ZnSO4.7H2O) and ammonium iron (II) sulphate or Mohr’s Salt [(NH4)2Fe(SO4)2·6H2O] were procured from Sigma-Aldrich and used without any further purification. Millipore water (18 MΩ) was used to prepare the solutions. Synthesis of ZnO/γ-Fe2O3 composite It has been reported previously [19] that a modified galvanic technique can be used for the deposition of ZnO thin films on fluorine doped tin oxide (FTO) coated glass substrates. This concept has been adopted to deposit the ZnO/γ-Fe2O3 composite films on FTO glasses in this work. Here, 100 ml of working electrolyte has been prepared by dissolving 0.287 g zinc sulphate heptahydrate and 0.113 g of Mohr’s salt in de-ionized water so as to make the final concentration of ZnSO4 and Mohr’s salt as 0.01 M and 0.004 M, respectively. The pH of the working solution was c.a. 6.8. Commercially available (Sigma-Aldrich) FTO coated glass (resistivity 7 Ω/sq.) was used as the cathode and a metallic Zn rod (99.8%) was used as the sacrificial anode for the galvanic deposition of the ZnO/γ-Fe2O3 film. The FTO coated glass substrates were cleaned by ultrasonication in acetone followed by boiling in anhydrous methanol for 15 minutes and then drying in air. After immersing the cleaned FTO glass and Zn rod in the working solution (containing both zinc and iron precursors), they have been connected externally by a conducting (Cu) wire. The distance between the FTO glass and Zn rod was 2 cm and the active area of the FTO substrate was 2.0 × 1.0 cm2. As soon as the FTO electrode was connected with Zn rod, oxidation of Zn takes place at the metal-electrolyte interface (E◦ox of Zn Zn2+ + 2e- is +0.76 V), resulting in Zn2+ ions which migrate to the solution. The electrons that were released on the 5 ACS Paragon Plus Environment

ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

zinc surface due to the oxidation of Zn to Zn2+, immediately migrate to the FTO electrode through the external conducting path and make the FTO surface electron rich. In the working electrolyte the Mohr’s salt dissociates to generate Fe2+ ions which, along with Zn2+ ions, migrate to the electron rich FTO electrode, i.e. cathode and there they get reduced to the respective metal hydroxides, as the pH of the working solution is almost neutral (c.a. 6.9). The almost equal ionic mobility of Fe2+ (µeff* = 9.93 × 109 m2S-1V-1) and Zn2+ (µeff* = 7.65 × 109 m2S-1V-1) helps in codeposition of the respective metal hydroxides in nearly 1:1 ratio. So, on the cathode surface, a film containing both iron and zinc hydroxides will be formed which is then converted to ZnO/γFe2O3 heterostructure on post deposition annealing at 600ºC for 15 minutes in air. The deposition was carried out for 6 hours at 30ºC without stirring. The following reactions can be proposed in support of the film deposition: In solution: ZnSO4 Zn2+ + SO4=

(1)

(NH4)2Fe(SO4)2 2NH4+ + Fe2+ + 2SO4=

(2)

H2O = H+ + OH-

(3)

At anode: Zn Zn2+ + 2e-

(E◦ox = + 0.76 V)

(4)

At cathode: Fe2+ + 2 OH- Fe(OH)2

(5)

Fe(OH)2 + OH- FeO(OH) + H2O + e-

(6)

Zn2+ + 2 OH- Zn(OH)2

(7)


Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

On annealing: Fe(OH)2 + FeO(OH) + Zn(OH)2 ZnO/γ-Fe2O3

(8)

The formation mechanism of iron oxyhydroxide (lepidocrocite, FeO(OH)) from faintly acidic Fe2+ solution has been discussed in detail elsewhere [20]. Characterization In order to determine the phase purity and structure of ZnO/γ-Fe2O3 composite X-ray diffraction (XRD, Philips PANalytical X’PERT Pro, parallel beam geometry, Bragg-Brentano goniometer, source Co Kα, λ = 1.78897 Å) technique has been used. The morphology of the ZnO/γ-Fe2O3 films was investigated by a field-emission scanning electron microscope (FESEM Carl Zeiss, Sigma). An in-depth structural analysis has been made using transmission electron microscopy (TEM, JEOL JEM 2100F). Detailed discussion on the morphology and other structural properties of the deposited films has been provided in the Electronic Supplementary Information file.

Selective sensing of hydrogen sulfide gas The gas sensing properties of the ZnO/γ-Fe2O3 heterostructure based sensor prototype, developed in this work were investigated in presence of three hazardous gases like methane (CH4), carbon monoxide (CO) and hydrogen sulfide (H2S). The concentration of each gas was maintained 1%, with balance nitrogen, throughout the experiments. The flow of all gases were measured and controlled with mass flow controllers and the flow rate of each gas was kept constant at 35 SCCM (standard cubic centimeter per minute) during the experiment. No external temperature was applied during the measurements and the sensor prototypes were exposed in 7 ACS Paragon Plus Environment

ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

gases only at room temperature (~30°C). After each measurement cycle, the chamber was cleaned with a gas suction pump and by purging with dry air. The developed ZnO/γ-Fe2O3 heterostructures showed significantly high response towards hydrogen sulfide at an operating temperature of 30°C. There are many previous reports on the sensing of H2S, however, in most of the cases [11,12,21,22], the operating temperature was notably higher, which is not desirable for the detection of H2S in the presence of other explosive/combustible gases like CH4. Even, commercially available MOS based H2S sensors also have higher operating temperature c.a. 250ºC. The room temperature selective detection will also help in facile fabrication of portable gadgets. Keeping these in mind, we aimed to develop a new material which would be capable of sensing H2S at very low operating temperature like 30ºC with significant response, selectivity and low response and recovery time. The response curves of the ZnO/γ-Fe2O3 based sensor prototype towards the different test gases are shown in Figure 1a. The sensor prototype has shown stable response of about 82.5% towards hydrogen sulfide at 30ºC, though; the average response was found to be about 80%. A drop in resistance of about 66 MΩ (from 80 MΩ to 14 MΩ) within 85 sec was observed and more than 80% sensing response was observed within first 60 sec of gas exposure with 80% recovery within 300 sec at 30°C. In presence of H2S, a drop of resistance of the ZnO/γ-Fe2O3 heterostructure indicates that the prototype has an n-type response towards H2S. On the other hand, an opposite trend was noticed when the prototype was exposed to both CH4 and CO. The base resistance was found to increase when the prototype was exposed to 1% CH4 and CO at the same operating temperature. A rise in resistance of about 15 MΩ (from 80 MΩ to 95 MΩ) was observed when the prototype was exposed to CO and 10 MΩ (from 80 MΩ to 90 MΩ) rise was noticed for CH4. In both cases, the ZnO/γ-Fe2O3 film was found to exhibit a p-type behavior with 8 ACS Paragon Plus Environment

Page 8 of 28

Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

negative sensitivities in comparison to H2S. The highest relative response of -19.25% was observed in case of CO, with an average relative response of -18.5%. For CH4, the highest relative response was about -13.21% with average relative response -12.8%. The n-type response characteristic of ZnO/γ-Fe2O3 in presence of H2S and its transition towards p-type response in presence of CO and CH4 has made the sensor prototype markedly selective towards H2S. This result seems to be very significant, since it will help us to overcome the longlasting problem of selective H2S sensing at low operating temperature. The response of this sensor prototype was stable and repeatable, for all the three test gases, which has been presented in Figure 1a and the response bar diagram is shown in Figure 1b. A switchover to p-type conduction is possible in certain condition using mainly ZnO/γ-Fe2O3 heterostructure which has been explained with a suitable sensing mechanism and energy band diagram in the section “explanation of selective sensing mechanism”. The concentration dependence of the proposed sensor prototype for H2S, CO and CH4 is shown in Figure 1c. The measurements were carried out within a concentration range of 0.25% to 1.0% of the three test gases. The sensor was found to be highly selective towards H2S over this wide range of concentration. The response of the sensor material was found to increase with an increase in the concentration of hydrogen sulfide, but this trend was not so prominent for CO and CH4. The order of H2S response was about 12(±1.8)>38(±2.5)>67(±3.52)>82(±4.28)% for, 0.25, 0.50, 0.75 and 1.0% of the gas, respectively. But, in the case of CO and CH4, the response was found negative over this wide range of concentration. Also, the trend of increasing negative relative response for CO and CH4 was not as prominent as H2S. The response was found as 2.5(±0.85)>-9(±1.0)>-15(±1.42)>-19(±1.93) for CO and -1.5(±0.5)>-6(±1.1)>-10(±1.22)>12.9(±1.51) in case of CH4, for 0.25, 0.50, 0.75, and 1.00% of respective gases. The results 9 ACS Paragon Plus Environment

ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

indicate that the proposed sensor prototype is quite eligible for the selective detection of H2S among the three test gases viz. H2S, CH4 and CO over a wide range of concentration at a significantly low operating temperature of 30ºC. This is well desired for the practical use of the sensor in confined areas like manholes and coal mines. To realize the repeatability and stability of the fabricated sensor prototype, total 40 cycles of measurements were carried out in presence of 1% H2S and the results are shown in Figure 1d. The sensor performance was found to be notably consistent throughout the 40 cycles, however, slight decrease in response was observed after 40 consecutive measurements, which might be attributed to the trapping of the test gas to some extent and less degassing during the last couple of cycles. The overall result was found highly acceptable. A comparison (Figure 2) has also been made on the performance of the fabricated sensor prototype with commercially available hydrogen sulfide sensor under the same gaseous environment. The procured sensor showed a response of 84(±3.5)% towards CH4, 87(±4.2)% towards CO and 91(±4.0)% towards H2S, without any selectivity. The operating temperature of the procured sensor was also significantly high, i.e. ~250˚C. The datasheet for the procured sensor has been provided as Electronic Supplementary Information. On the other hand, the sensor prototype reported in this work showed 82(±4.28)% response selectively towards H2S at an operating temperature of 30˚C. This indicates high selectivity and response of the fabricated ZnO/γ-Fe2O3 heterostructure even at far lower operating temperature towards H2S in comparison to the commercially available metal oxide based sensor. The response of only Fe2O3 towards H2S can be seen from Electronic Supplementary Information.


Page 10 of 28

Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

However, the recovery time reported here is relatively higher than the response time of the device. Normally this is observed for the MOS based gas sensors working at low operating temperature (as reported here, 30°C). The probable explanation and solution to this may be as follows: During recovery of a metal oxide based sensors two parallel processes take place on the surface:

a) Removal of adsorbed gas molecules from the surfaces, where the adsorbed gas molecules releases to the atmosphere b) Re-adsorption of atmospheric oxygen on the sensor surface either in molecular or in ionized form.

Rate of adsorption and desorption of gas molecules on the surface of the sensing materials depends on the surrounding temperature. Both the processes are quite time consuming at low operating temperature. In this paper the response temperatures of the sensor is very low (30°C). This could be the probable reason of higher recovery time with respect to the response time. It may be improved by increasing the operating temperature; however the sensor will lose its selectivity and the power consumption will increase. Only way to improve the recovery time is to use MEMS (Micro-Electro-Mechanical-System) microheater based sensing platform which will be controlled with microcontroller. In that case two different operating temperatures are required, one for the response and another for the recovery of the sensor. The controller is required programming in such a way that the sensor will be heated up at its required lower operating temperature during response time and it will go for higher pulse to recover at higher temperature.


ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Explanation of selective sensing mechanism The fundamental sensing mechanism of the ZnO/γ-Fe2O3 is based on the change of resistivity of its surface in the presence of the test gas molecules, which is primarily governed by the chemisorption of oxygen molecules on the material surface from the ambient. Basically, ZnO/γ-Fe2O3 has an electron rich surface and one oxygen molecule from the ambient attracts one electron from the conduction band of the composite to form O2-. This reaction takes place at room temperature (30ºC) and the O2- ions prevails in the surface up to 150ºC. When the target gas molecules come at the surface, they may react either with the semiconductor or with the adsorbed O2- ions, which brings a change in the number of free electrons in the semiconductor surface, as well as the electrical conductivity of the semiconductor. It is well known that the iron (III) oxide (i.e. Fe2O3) is used in industries either as an absorbent or as catalysts for H2S removal from different gaseous mixture at low temperature (20-200°C) and as well as at high temperature (300-800°C) [23]. Removal of H2S mainly occurs at low operating temperature due to the solidgas interactions in a thin hydrated lattice of Fe2O3. H2S breaks to form H+ and S2- in the presence of Fe2O3 even at 30°C [23-26]. Here γ-Fe2O3 acts as the catalyst for this reaction and reduces the H2S gas at the surface of the sensing layer. Some intermediate reactions can also take place between Fe2O3 and H2S to form FeS/FeS2/Fe2S3 and as a byproduct; H2O is released [23-26]. Again, the electronegativity of oxygen (3.44) is much higher than that of sulfur (2.58) and this indicates that the formed FeS/FeS2/Fe2S3 will again react with the active oxygen species i.e. O2¯ and will regenerate the Fe2O3 [23]. The released ionized sulfur will further react with the O2¯ ions present in the semiconductor surface to produce SO2 as the byproduct. In both the reactions, O2¯ ions release a huge number of electrons at the surface of the sensing layer. As a result, the resistance of the ZnO/γ-Fe2O3 sensor material decreases rapidly and a good n-type response is 12 ACS Paragon Plus Environment

Page 12 of 28

Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

observed in the presence of the gas H2S. On the other hand, for carbon monoxide and methane, the ZnO/γ-Fe2O3 composite is unable to break the gases at the lower temperature, though they are adsorbed on the sensing layer by dragging electrons from the surface [27-29]. Hence the numbers of electrons are reduced on the sensing surface which leads to an increase in surface resistance and the response of the sensing material looks like p-type in nature. The probable sensing mechanism of the ZnO/γ-Fe2O3 film has been explained in details with the help of Scheme 1(a-c). Here, ZnO also plays a crucial role to enhance the response of the sensor prototype. Primarily, ZnO has been used as the backbone to develop the flower like structure of the sensing film. This particular type of structure increases the surface area and roughness, as well as surface-to-volume ratio, leading to better adsorption of the gas molecules on the surface of the composite film. As a result, the amount of the reactive oxygen species viz. O2¯ ion increases on the surface, which in turn enhances the response of the sensor prototype. However, the extent of formation of O2¯ ions on the ZnO surface will be lower. As the ZnO/Fe2O3 straddling heterojunction contains more charge than bare ZnO surface, major amount of O2¯ ions will be formed in the junction at 30°C (has been elaborately discussed in the coming sections). Therefore, the amount of gas adsorbed on the ZnO surface will have less impact in the sensing performance. The response of the composite material may also be described by considering the band structure, band bending, change of work function and electron affinity. In this case, the band gap of n-type γ-Fe2O3 (Eg = 2.1 eV) is smaller than that of n-ZnO (Eg = 3.2 eV). Again, the electron affinity of γ-Fe2O3 (χ = 4.7 eV) is larger than that of ZnO (χ = 4.1 eV), which indicates that these two semiconductors can create an N-n straddling type heterojunction at the interface [15,30]. The energy band diagrams of ZnO/γ-Fe2O3 N-n straddling heterojunction before and after exposure to 13 ACS Paragon Plus Environment

ACS Sensors 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

different gases have been presented in the Scheme 2. The individual energy-band diagrams of ZnO and γ-Fe2O3 are shown in Scheme 2a. At the interface of ZnO and γ-Fe2O3, a barrier (q∆Vs) is formed by band bending as shown in Scheme 2b. Conduction mechanism at the surface of a semiconductor again depends on the mobility of the free charge carriers at the time of sensing a gas. Primarily, the free charge carriers need to overcome the barrier height appearing at the interface of the two metal oxides to increase the conductivity. If the barrier height decreases, the movement of the electrons would be easier. As a result, the mobility of the electrons will increase leading to an increase in conductivity. The barrier height changes with the adsorption or desorption of the gas molecules which will also put its signature on the work function (Φ) and conduction behavior of the composite material. Mainly the change in work function depends on the three major parameters like electron affinity (χ) of the individual semiconductors, band bending (q∆Vs) and the difference between Fermi level and the conduction band [(EC ̶ EF)bulk] at the bulk [31], which may be expressed as: Φ = (EC - EF)bulk + qVS + χ

(9)

According to earlier reports [29,31], the conductivity of a surface having two different materials which are polycrystalline in nature, arises from the electrons and holes present in the surface and may be expressed as,

‫ܩ‬ௌ =

೜൫ೇೄ೚ షೇೄ ൯ ൠ ಼ಳ ೅

ீೄ೔ ௘௫௣൜ ଶ

൥

೜൫ೇೄ೚ షೇೄ೔ ൯ ൠ ಼ಳ ೅

௘௫௣൜

+

೜൫ೇೄ೚షೇೄ೔ ൯ ൠ ಼ಳ ೅

௘௫௣൜

௘௫௣൜

೜൫ೇೄ೚ షೇೄ ൯ ൠ ಼ಳ ೅

൩

(10)

Where G and Gsi are the conductance and intrinsic conductance of the surface, Vso, Vsi and Vs are the potential barriers at the interface of two granular molecules at initial condition


Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sensors

(before gas charge), intrinsic condition (equal contribution of holes and electrons) and test condition (after gas charge), respectively. KB is the Boltzman’s constant and T is the temperature. Now, in the equation (10), exp{q(Vso-Vsi)/KBT} is invariant as all the parameters are constant for a particular material. If exp{q(Vso-Vsi)/KBT} >> 1 and (Vso-Vsi) = + ve, the density of holes is higher than the density of electrons and the surface shows a p-type conductivity. If exp{q(Vso-Vsi)/KBT} = 1 and (Vso-Vsi) = 0, the density of holes is equal to the density of electrons and the surface shows an intrinsic type conductivity. Again, if exp{q(Vso-Vsi)/KBT}

Î³-Fe2O3 Charge Transfer Interface toward ... - ACS Publications

Recommend Documents